The development and testing of advanced gas turbine engines requires the availability of sensors capable of accurately measuring temperatures in the range of about 980.degree. C. (about 1,800.degree. F.) and higher. Such sensors must be able to withstand rapid temperature cycling and complex gas flows. They must also be compatible with the component to which they are attached and must conform to the component surface without significantly disturbing air flow patterns over the component.
In commonly assigned U.S. Pat. No. 3,890,456 to Dils, a method for forming such a sensor on a superalloy gas turbine engine component is disclosed. As shown in FIG. 1, the Dils sensor 10 comprises several layers of different materials applied to the surface 12 of a component 13 which is to be tested. The first layer 14 has an MCrAlY composition, where M is iron, cobalt, or nickel. A thermally grown layer 16 of aluminum oxide, formed by oxidizing the MCrAlY layer 14 lies over the MCrAlY layer 14 and acts as an electrical insulator. Separate noble metal layers 20, 22 such as platinum and a platinum-10% rhodium alloy respectively, lie over the thermally grown aluminum oxide layer 16 and are applied in such a manner that they form a junction 17. Lead wires are attached to each noble metal layer 20, 22 by, e.g., tack welding. The lead wires are then connected to a read-out device in conventional fashion.
One problem which limits the usefulness of sensors 10 applied by Dils is that the noble metal layers 20, 22 spall or become delaminated from the aluminum oxide layer. Attempts to improve the bond strength between the noble metal layers 20, 22 and the aluminum oxide layer 16 have not been entirely satisfactory; for example, grit blasting the aluminum oxide layer 16 prior to application of the noble metal layers 20, 22 has had limited success, and is even undesired since the rough surface of the aluminum oxide 16 sometimes results in poor electrical properties. Furthermore, the Dils' sensor 10 is particularly prone to failure if the noble metal layers 20, 22 are applied in a high humidity environment.
Consequently, what is needed is a thin film sensor with improved properties, particularly with improved strength between the noble metal layers and the insulating layer.